Apparatus for spectral analysis and counting of pulses generated from radio active disintegrations in a radiation detector

ABSTRACT

An apparatus for spectral analysis and counting of electrical pulses in an instrument for measuring radioactive disintegrations. The apparatus comprises a number of discriminators having preselected amplitude transmission ranges and optical indication means for indicating the spectral energy distribution of the pulses. Scalers are provided for counting the pulses within a number of these preselected amplitude ranges.

D United States Patent 1 1 1 3,772,515

Soini Nov. 13, 1973 [54] APPARATUS FOR SPECTRAL ANALYSIS 2,685,0277/1954 Alvarez 328/116 AND COUNTING 0F p s s GENERATED 12; Karl: e on 1FROM RADIO ACTIVE DISINTEGRATIONS 2,776,377 1/1957 Anger 250/833 R IN ARADIATION DETECTOR [75] Inventor: Erkki Juhani Soini, Turku, FinlandPrimary ExaminerI-Iarold A. Dixon [73] Assignee: Wallac OY, Turkulo,Finland A"0mey Amo|d Christen et [22] Filed: Mar. 28, 1972 [2]] Appl.No.: 238,892

[57] ABSTRACT [30] Foreign Applicafion Priority Data An apparatus forspectral analysis and counting of Apr. 16, 1971 Sweden electrical pulsesin an instrument for measuring radioactive disintegrations. Theapparatus comprises a [52] US. Cl. .0250566, number of discriminatorshaving presel cted a [5 CI. tude transmission ranges and opticalindication means Fleld of Search for indicating the spectral energydistribution of the 307/235 pulses. Sealers are provided for countingthe pulses within a number of these preselected amplitude [56]References Cited ranges UNITED STATES PATENTS 2,820,896 H1958 Russell eta1 328/116 3 Claims, 3 Drawing Figures on cm E211 lC3n Rn Gn-1 Dn-1Cln-l C2n-l C3n-l e 8 In -1 Rn-l 62 D2 C12 C22 C32 I2 R2 D1 61/ C1 1 C21C31 e |1 Cml Cm2 Cm3 Sc1 Sc2 Sc3 PATENTEU HEY I 3 I975 SHEET 2 OF 3Fig.2

PAIENIEmnv 13 ms SHEET 3 UF 3 MQE 2x88 82 P3 8 8 m 1 a 2 N I I l I I l Ii l l I l l l I l l I l l l l I l. l l I I l l l| |l O 0 28 N8 ,8 Q l II I l l l I I I I l I l I I I I l I I I I l I I I l l I I ||l 88 NS .8 ollllllllllllllllllllllllllllllll I... o 0 85 N6 5 {II 5 N.

APPARATUS FOR SPECTRAL ANALYSIS AND COUNTING OF PULSES GENERATED FROMRADIO ACTIVE DISINTEGRATIONS IN A RADIATION DETECTOR The presentinvention refers to an apparatus for spectral analysis and counting ofelectrical pulses generated by radioactive disintegrations in aradiation detector.

When measuring the radiation from a sample comprising for instance aB-radiating isotope, a liquid scintillation counter could be used. Thesample is dissolved in a scintillation liquid, which generates lightpulses when subject to B-radiation.The dissolved sample is disposedclose to one or several photomultipliers in which the light pulses giverise to electrical signals, the amplitudes of which indicate the energyof the B-radiation. A certain isotope will then give rise to an energyspectrum located between and a certain characteristic maximum energy.Thus for instance tritium gives a spectrum between 0 and 18 keV, carbon14 a spectrum between 0 and 155 keV and phosphorus 32 a spectrum below1800 keV. When counting the pulses obtained from the photomultiplier itis important that all pulses within the characteristic energy range ofthe isotope are counted whereas pulses having a higher energy, forexample obtained from the cosmic radiation, are suppressed.

When determining the amplitude interval within which the electricalpulses are to be counted one usually uses discriminators,the thresholdvalues of which are adjustable by means of linear potentiometers. Thethreshold level will thus be a linear function of the setting of thepotentiometer, which means that the relative accuracy will varysubstantially within the very wide range within which the potentiometerhas to operate due to the big differences between the maximum energy ofdifferent isotope (compare for instance tritium and phosphorus). Thediscriminators therefore have to be provided with amplitude rangeswitches or a logarithmic amplifier connected between the output of thephotomultiplier and the discriminators. These additional devices forcompensating the relative sensitivity are however expensive and does notin general give a complete compensation. A further drawback involved inthe above discriminators, where the threshold values are set analogouslyat the beginning of each series of measuring experiments, is that it isdifiicult to find the upper limit of the spectrum derived from thesample. As indicated above, the energy spectrum of a certain isotopecomprises the range from zero to a certain maximum value. Pulses willhowever be obtained above this maximum value due to radiation derivedfrom exterior radioactive sources. In order to determine the level belowwhich the pulses are to be counted one usually uses two discriminatorshaving adjustable threshold levels. By means of these two levels anarrow amplitude interval could be defined and by varying the thresholdlevels this interval could be swept through the complete amplituderange. The number of pulses per unit of time within this narrow intervalis then counted as the interval is swept from zero towards higherenergies and when the number of pulses does not reach a certain valuethe corresponding amplitude interval is considered to be the upper limitof the energy spectrum. It is realized that this method is very timeconsuming and gives a rather vague definition of the spectrum limit.

It is an object of the present invention provide an apparatus in whichthe above stated drawbacks of the threshold value adjustment areeliminated and in which one can obtain a substantially constant relativethreshold level accuracy within an arbitrary amplitude range. Thecharacteristics of the invention will appear from the claims attached tothe specification.

The invention will now be explained in detail, reference being made tothe enclosed drawing in which:

FIG. 1 shows a schematic block diagram of an apparatus according to theinvention FIG. 2 shows a portion of a matrix included in the apparatus,and

FIG. 3 shows the part of the apparatus that is visible from the frontpanel of the instrument.

Referring now to FIG. 1 reference PM denotes the input of the apparatus,this input being connected to the photomultiplier of the instrument. Theinput PM is connected to the input of a number of discriminators D1, D2Dn. Each discriminator might for instance consist of a biasedoperational amplifier, having a certain threshold value, only signalshaving an amplitude exceeding this value giving rise to an output signalfrom the respective discriminator. These threshold values of thedifferent discriminators should preferably be increasing in accordancewith an exponentional function. The outputs of the discriminators areconnected to one input of an end-circuit G1, G2 Gn-l respectively. Thesecond inverting input of each end-circuit is connected to thediscriminator having the next higher threshold level, an output signalthus being obtained from only one of the end-circuits when a pulse issupplied to the input of the apparatus. This output signal will thusindicate the highest threshold level exceeded by the pulse. The outputsof the end-circuits are connected via conductors R1, R2 Rn to suitableindicating devices I1, I2 In,e.g., consisting of lamps. The lightintensity of the lamps will thus give visible information about thespectral distribution of the output signals obtained from thephotomultiplier. The apparatus of FIG. 1 further comprises three scalersScl, Sc2 and Sc3. The inputs of these scalers are connected toconductors Cml, Cm2 and Cm3 perpendicular to the conductors R1 Rnconnected to the outputs of the discriminators, the scaler conductorsand the discriminator conductors intersecting each other at crossingpoints C11, C12, Cln, C21, C22, C2n and C31, C32, C3n respectively. Atthese crossings the conductors are connectable to each other as will beexplained below. In each sealer one will thus obtain the sum of thenumber of pulses having an amplitude within the amplitude range of theamplitude channels connected to the conductor of the respective scaler.FIG. 2 illustrates how the connections between the conductors R1 Rn andCml Cm3 could be carried out in practice. The conductors R1, R2, Rn andCml Cm3 respectively are arranged perpendicular to each other onopposite sides of an insulating plate Pl. At each point of intersectionC11 C31, C12 C32 there is a bore through the plate and the conductors.An electronic contact between the conductors can be provided by meanssuch as a pin Kn comprising two sections of conducting material whichare brought into contact with the respective conductors, the conductingsections being interconnected via a diode in order to prevent pulses onthe parallel conductors to affect each other.

The function of the apparatus will now be explained in connection withFIG. 3.

In FIG. 3 which shows the part of the apparatus according to FIG. llvisible from the front panel of the in strument, references Ill, l2, I33denote the lamps having the corresponding references in FIG. I. Theapparatus is thus supposed to comprise 33 amplitude channels. Under thelamps there are three horizontal rows of holes, each hole correspondingto a crossing point of the matrix in FIG. 1 and having the samereferences as in FIG. 1. By inserting a plug or pin Kn, in a respectivehole, the intersecting conductors are brought into electrical contactwith each other. The front panel is further provided with a keV diagram,in which the limits of the respective amplitude channels are indicated.Thus for instance a pulse within channel 2 corresponds to a radiationenergy of the radio active sample between 1.26 and 1.58 keV. Over thescale the approximate energy distributions for tritium, carbon 14 andphosphorus 32 are indicated. A sample comprising for instance theradioactive isotope phosphorus 32 will thus give rise to a lightintensity spectrum of the lamps 11-133 which approximately correspondsto the curve denoted 32 P. By means of the lamps a visible indication ofthe spectral distribution of the sample is thus obtained and one caneasily determine if the sample is normal or if there are reasons tosuspect defects of the sample or the apparatus. From the indications ofthe lamps it is also easy to determine the upper threshold limit of thesample and it will not be necessary to make a time consuming sweepacross the complete amplitude range as in the prior art designs. Whenthe spectral distribution of the sample has been determined from theintensities of the lamps plugs are inserted in the holes correspondingto the amplitude interval of the amplitude range to be monitored, thesum of the number of pulses within this interval being counted in thescaler corresponding to the horizontal row where the plugs are inserted.

The reason why the apparatus is provided with three counters and threehorizontal rows of holes is that certain specimens comprise more thanone isotope, so called double labelled or triple labelled samples. Theenergy spectrum of the different isotopes of such samples could beapproximately measured simultaneously by connecting the differentsealers to different amplitude ranges and by making certain calculationsof the sums obtained in the scalers.

We claim:

ll. Apparatus for spectral analysis and counting of electrical pulsesgenerated by radioactive disintergrations in a radiation measuringinstrument, the apparatus comprising a number of discriminators, eachdiscriminator transmitting signals within a specific preselectedamplitude range, these amplitude ranges being located adjacent to eachother, the inputs of the discriminators forming the input of theapparatus, a number of optical indication means, the outputs of therespective discriminators each being connected to one of said means,these means indicating the instantaneous spectral amplitude distributionof the pulses, the apparatus further comprising a number of sealers,connectable to a desired number of discriminator outputs, these outputsbeing chosed with respect to the indicated amplitude distribution, anumber of conductors connected to the discriminators and scalersarranged to form rows and columns respectively in a matrix, the rows andcolumns being connectable at their intersecting points, and means toselectively connect a conductor of a discriminator with the conductor ofa sealer at an intersection of a row and a column.

2. Apparatus according to claim 1, characterized in that the width ofthe specific amplitude ranges increases as the amplitudes within theranges increase.

3. Apparatus according to claim 1, characterized in that the indicatingmeans consist of lamps.

=1 t i =l= =1

1. Apparatus for spectral analysis and counting of electrical pulsesgenerated by radioactive disintergrations in a radiation measuringinstrument, the apparatus comprising a number of discriminators, eachdiscriminator transmitting signals within a specific pre-selectedamplitude range, these amplitude ranges being located adjacent to eachother, the inputs of the discriminators forming the input of theapparatus, a number of optical indication means, the outputs of therespective discriminators each being connected to one of said means,these means indicating the instantaneous spectral amplitude distributionof the pulses, the apparatus further comprising a number of scalers,connectable to a desired number of discriminator outputs, these outputsbeing chosed with respect to the indicated amplitude distribution, anumber of conductors connected to the discriminators and scalersarranged to form rows and columns respectively in a matrix, the rows andcolumns being connectable at their intersecting points, and means toselectively connect a conductor of a discriminator with the conductor ofa scaler at an intersection of a row and a column.
 2. Apparatusaccording to claim 1, characterized in that the width of the specificamplitude ranges increases as the amplitudes within the ranges increase.3. Apparatus according to claim 1, characterized in that the indicatingmeans consist of lamps.